This invention relates to a cathode ray tube such as a color cathode ray tube and, more particularly, it relates to the state of the high resistance conductive film applied to the inner wall surface of the neck of a cathode ray tube.
Generally, a color cathode ray tube comprises an envelope having a panel, a funnel and a neck constructed as integral parts thereof. The panel carries on the inner surface thereof a fluorescent screen (target) having three stripe-shaped or dot-shaped fluorescent layers that fluoresce respectively in blue, green and red. The panel also has therein a shadow mask provided with a large number of apertures and arranged vis-a-vis the fluorescent screen.
The neck contains therein an electron gun assembly. The electron gun assembly is adapted to emit three electron beams that proceed on a same horizontal plane and include a center beam and a pair of side beams. The three electron beams emitted from the electron gun assembly are converged toward the fluorescent screen and focused on the respective fluorescent layers of blue, green and red.
A deflection yoke arranged on the outside of the funnel produces a non-uniform magnetic field for deflecting the three electron beams emitted from the electron gun assembly in horizontal and vertical directions. Thus, the three electron beams emitted from the electron gun assembly are forced to scan the fluorescent screen both horizontally and vertically by way of the shadow mask by the non-uniform magnetic field. As a result, a color image is displayed on the screen.
Referring to FIG. 1 of the accompanying drawings, the color cathode ray tube has an internal conductive film 7 formed on the inner surface of the envelope and extending from the funnel to the neck 3. The internal conductive film 7 is electrically connected to the anode terminal arranged on the funnel. On the other hand, the convergence electrode 9 of the electron gun assembly 8 is electrically connected to the internal conductive film 7 by way of a bulb spacer 10. Thus, the anode voltage supplied from the anode terminal is applied to the convergence electrode 9 by way of the internal conductive film 7 and the bulb spacer 10.
However, in a color cathode ray tube having a configuration as described above, the converging performance of the three electron beams may change as the electric potential of the inner wall surface of the neck 3 changes with time. As a result, the three electron beams may not land on the respective fluorescent layers to give rise to a problem of color deviations in the displayed image.
More specifically, the problem occurs in the following manner.
Since the neck is made of an insulator material, or glass for instance, it is apt to become electrically charged and then discharge the accumulated electric charge. Therefore, the potential of the electric charge of the inner wall surface of the neck, i.e. the neck potential, comes to show a predetermined potential distribution pattern immediately after the application of the anode voltage under the influence of various components including the internal conductive film 7 and the convergence electrode 9 of the electron gun assembly 8.
However, as time goes on, stray electrons generated within the neck eventually collide with the inner wall surface of the neck, thereby causing secondary electrons to be emitted from the inner wall surface, and gradually raise the neck potential. As a result, the neck potential changes with time.
The neck potential affects the electric field operating as main electron lens section of the electron gun assembly. Then, as the neck potential is not stably held to a constant level but rises with time, it gradually but remarkably permeates into the electric field of the main electron lens section. Thus, in the course of time, the neck potential changes the distribution of the electric field operating as main electron lens section. Since the neck potential permeates into the main electron lens section from the periphery thereof, it alters the tracks of the two side beams passing through a peripheral area of the main electron lens section.
Thus, color deviations occur in a color cathode ray tube adapted to emit three electron beams because of the phenomenon of the change with time of the converging performance of the electron beams, which is referred to as convergence drift.
Japanese Patent Applications KOKAI Publication Nos. 64-12449 and 5-205560 propose the use of a high resistance conductive film 17 having a coefficient of electron emission smaller than one and arranged on the inner surface of the neck as shown in FIG. 1. The high resistance conductive film 17 is directly arranged on the inner wall surface of the neck and held in contact with the internal conductive film 7. As a result, it can prevent the change with time of the neck potential due to the emission of secondary electrons of the neck and suppress color deviations due to convergence drift.
However, when a high resistance conductive film is arranged on the inner surface of the neck and held in contact with the internal conductive film in a manner as described in Japanese Patent Applications KOKAI Publication Nos. 64-12449 and 5-205560 and if the high resistance conductive film has a uniform film thickness as seen from FIG. 1, a problem arises as described below.
Referring to FIG. 1, if the central axis of the neck which is the axis of the tube is Z-axis, the resistance of the high resistance conductive film 17 per unit length of the Z-axis is constant. Additionally, since the neck potential is relatively high if compared with its counterpart of a cathode ray tube having no high resistance conductive film 17, a phenomenon of field emission is apt to occur between any metal part of the electron gun assembly 8, which may be an electrode, and the inner wall surface of the neck to give rise to a problem of reduced withstand voltage.